Rabbit BL2020, User Manual

Page: 26 / 94

22 Wildcat (BL2000)
3.4 Serial Communication
The BL2000 has one RS-232 serial channel (with RTS/CTS) or two RS-232 (3-wire)
channels, one RS-485 serial channel, and one CMOS serial channel. The RS-232 chan-
nel(s) are configured with the
serMode
software function call. Table 2 summarizes the
options.
All four serial ports operate in an asynchronous mode. An asynchronous port can handle 7
or 8 data bits. A 9th bit address scheme, where an additional bit is sent to mark the first
byte of a message, is also supported. Serial Port A can be operated alternately in the
clocked serial mode. In this mode, a clock line synchronously clocks the data in or out.
Either of the two communicating devices can supply the clock. The BL2000 series boards
typically use all four ports in the asynchronous serial mode. Serial Ports B and C are used
for RS-232 communication, and Serial Port D is used for RS-485 communication. The
BL2000 uses an 11.0592 MHz crystal, which is doubled to 22.1184 MHz. At this fre-
quency, the BL2000 supports standard baud rates up to a maximum of 230,400 bps.
3.4.1 RS-232
The BL2000 RS-232 serial communication is supported by an RS-232 transceiver, U1. U1
provides the voltage output, slew rate, and input voltage immunity required to meet the
RS-232 serial communication protocol. Basically, the chip translates the Rabbit 2000’s
CMOS/TTL signals to RS-232 signal levels. Note that the polarity is reversed in an
RS-232 circuit so that a +5 V output becomes approximately -10 V and 0 V is output as
+10 V. U1 also provides the proper line loading for reliable communication.
RS-232 can be used effectively at this baud rate for distances up to 15 m.
3.4.2 RS-485
The BL2000 has one RS-485 serial channel, which is connected to the Rabbit 2000 Serial
Port D through U8, an RS-485 transceiver. U8 supports the RS-485 serial communication
protocol. The chip’s slew rate limiters provide for a maximum baud rate of 230,400 bps,
which allows for a network of up to 300 m (or 1000 ft). The half-duplex communication
uses the Rabbit 2000’s PB6 pin to control the transmit enable on the communication line.
The BL2000 can be used in an RS-485 multidrop network. Connect the 485+ to 485+ and
485– to 485– using single twisted-pair wires (nonstranded, tinned) as shown in Figure 12.
Note that a common ground is recommended.
Table 2. Serial Communication Configurations
Mode
Serial Port
B C D
0 RS-232, 3-wire RS-232, 3-wire RS-485
1 RS-232, 5-wire CTS/RTS RS-485